1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same, in particular to a thin film transistor (TFT) provided on an amorphous transparent substrate.
2. Description of the Related Art
In recent years, high-definition display has been realized by forming a polycrystalline semiconductor TFT (in particular polysilicon TFT (p-Si TFT) on an alkali-free glass substrate. In the most popular technique for forming a p-Si film which serves as an operative semiconductor film of the p-Si TFT, an amorphous silicon (a-Si) film is formed first, and the film is then irradiated with a UV short-pulse excimer laser light. This allows only the a-Si film to fuse and crystallize without adversely affecting the glass substrate, and thereby the p-Si film is obtained.
[Patent Document 1]
U.S. Patent Application Publication No. 2002/0031876A1
[Patent Document 2]
Japanese Patent Application Laid-Open No. 10-173192
[Patent Document 3]
Japanese Patent Application Laid-Open No. 2002-33481
[Non-Patent Document 1]
2001 AM-LCD p. 243
A high-power, linear-beam excimer laser adapted to wide-area irradiation has been used in order to form the p-Si film based on the above-described technique, and a top-gate-type thin film transistor using thus-crystallized, p-Si film has also been developed. In the excimer laser crystallization, crystal grains isotropically grow from randomly-generated nuclei, only to achieve a grain size of as small as less than 1 μm. This results in a large population of small-sized crystal grains in the channel region, and in a mobility of the TFT of typically as small as 150 cm2/Vs or around. This value might be approximately 100 times larger than the mobility of a-Si MOSFET, but is approximately one-fourth of that of single-crystal silicon MOSFET (Si-MOSFET). The mobility will become larger in the channel portion having a larger grain size and a less amount of grain boundary extending along the direction of the length of the channel, and will become smaller in the channel portion having a smaller grain size and a larger amount of grain boundary extending along the direction of the length of the channel. The grain boundary has a lot of defects, and presence of the boundary within the channel portion will suppress the characteristics from being fully exhibited. In order to realize a high gm (large ON-current) with a polycrystalline semiconductor film, it is therefore necessary to increase the grain size, or to adopt some new design of TFT structure capable of achieving a high gm.